1. Technical Field
This disclosure relates to control circuits for lighting systems. In particular, it relates to control circuits for LED lighting systems with a feedback loop to regulate a drive voltage for the lighting system.
2. Background
Light-emitting diodes (LED) are semiconductor light sources traditionally used as indicator lamps in many devices. In addition, LEDs are increasingly also being used for lighting, where one particular use is for providing backlighting. For example, LED backlighting is increasingly being used for liquid crystal displays (LCDs), as LCDs do not produce their own illumination. Furthermore, LED backlight lighting systems are becoming increasingly common for the use in display backlighting and keypad backlighting in portable devices such as cell phones, smartphones, PDAs, digital cameras, personal navigation devices and other portable devices with keypads and/or LCD displays.
LED lighting systems are generally associated with a variety of advantages over traditional lighting sources such as incandescent lighting. For example, LEDs are efficient, associated with longer life, exhibit faster switching and produce less heat than traditional lighting sources. Due to the faster switching characteristics of LEDs, they are suitable for use in fast and highly responsive circuits by allowing for both quick response/start-up time and the capability to be operated at high frequency, further allowing for such enhancements as frequency modulation in order to reduce power consumption.
LED lighting systems typically comprise “strings” of stacked LEDs in which multiple LEDs are connected in series. Therefore the LED driver control circuit has to be able to provide a regulated high supply voltage. A common practice is to pull a well-defined current from the bottom of each LED string, via current sources or resistors and regulating the voltage across them. In such a way the power dissipation across the current sources can be minimized. In order to protect the system components from excessive voltage levels and avoid excessive high current to flow in the LED circuit, an overvoltage protection mechanism is generally provided to disable the delivery of power to the circuit in the event that the voltage rises above a certain threshold.
In the LED lighting systems described above (“strings” of stacked LEDs in which multiple LEDs connected in series), if one of the many individual LEDs in an individual LED string fails, an open-circuit condition for the entire associated LED string can occur. In such a condition the feedback mechanism employed to regulate the drive voltage generally causes the drive voltage to be further increased up to the point where the overvoltage protection circuitry disables the entire lighting system.
LED lighting systems can have many LED strings, for example five, six or even thirty or more. Consequently, if an open-circuit condition occurs in any one of the LED strings, the entire lighting system becomes inoperable due to the overvoltage protection mechanism. While this solution does successfully protect the circuits from the excessive currents associated with an overvoltage condition, the entire circuit and all the LED strings become unusable if there is an open-circuit condition in only one of the LEDs of one of the plurality of LED strings. Thus, there is a need for a fault-tolerant controller that is capable of providing overvoltage protection without disabling the entire LED lighting system in the case of a failure of an LED in one of the LED strings.